For More Information

This is the first report about travel time reliability performance measures at a national level.

The report describes congestion problems in 328 seriously congested corridors in the US over a variety of times – all day, morning and evening peaks, midday, weekends.

This report shows that much of our national congestion problem exists in a relatively small amount of our system.

Why do corridors vary from about 3 miles to about 40 miles? How were these chosen/defined?

We let the data tell these stories; we investigated all freeways and highways in the United States looking for traffic problems. A short directional roadway segment (less than 1 mile) with congestion for more than 10 hours in a week was the beginning of a congested corridor. (“Congestion” was having a speed less than half of the free-flow speed). Each directional, adjacent and upstream segment of roadway that was congested for 4 hours per week was included in the corridor. Four hours was chosen as the threshold after reviewing the data which showed that many upstream segments had some congestion nearly every weekday. Since it typically did not constitute every day of the week, choosing four hours allows one day per week to have a different queuing pattern. A minimum corridor length was set at 3 miles. This resulted in 328 directional freeway corridors. We combined traffic volume information from the states with the speed data to compute the performance measures along these corridors.

In some cases, the longer corridors could contain several bottlenecks that all run together.

What causes the reliability measures to increase (or said another way, what makes reliability worse?)?

Reliability measures climb when travel time variability increases. Every occurrence of an unpredicted travel disruption creates slower speeds than normal and contributes to an increase in the reliability measures.

Why do some of the worst congested corridors (Table 2) have much better rankings for reliability and vice versa?

There are several basic contributors to congestion: demand, crashes, weather, work zones, and special events. Some sections of road where demand is much greater than capacity for much of the day can actually be relatively reliable; it is hard to slow down traffic that is already slow. But where traffic is relatively light most of the day, disruptions to the traffic flow can have a much larger effect on the day’s travel conditions and increase the variability in corridor travel times.

What sorts of solutions address the lack of reliability? Can reliability issues be solved with adding capacity or mainly through operations/management?

Capacity can definitely help. It can help with recurring delay (providing new capacity to offset very high normal daily demand) but additional capacity can also keep a portion of the roadway operating if an incident blocks a lane.

Operational treatments can help to reduce some of the variability.

Freeway ramp meters help to maintain a constant speeds on the freeway mainlanes during congested periods (reducing variability)

Service patrols help to clear incidents quicker, which helps to reduce travel time variability.

Changeable message signs keep the motorists informed of incidents before the motorist becomes affected by them (avoiding potential delay and added variability)

Advanced traffic management regulates the roadway speeds similar to ramp metering; traffic may be able to maintain a constant flow (thus reducing variability in travel times).

Why is reliability more of a problem around bridges, tunnels, tolled facilities?

Reliability becomes more of an issue around constraints such as bridges because the capacity is at a premium at those locations. There may be few alternate routes and any small incident can have a huge effect on the corridor travel times. Variability tends to be higher where the roadway capacity is constrained like this.

Same can be said of freeways without shoulders. If an incident occurs in an area with no shoulders it will block a lane or multiple lanes and increase the travel times through the corridor. The opportunity for a lane blocking event goes way up when no shoulders are present.

Do treatments differ for corridors with greater weekend or midday delay than those with traditional peak period congestion?

The need for operational or management treatments may be greater in corridors where special events occur because the traffic patterns are often very different than those that occur during the typical weekday peak periods. Often the motorists that may be at a special event on a weekend may not be accustomed to driving in a particular corridor and may drive more cautiously and may need more guidance to navigate the area.

What role do the trucks play in congestion and reliability in these corridors?

Trucks are part of the traffic stream and are affected just like all motorists. Some truck drivers have the option of traveling in the midday and overnight periods to avoid driving in the peak periods. This depends on delivery schedules – manufacturers and delivery companies do not always this flexibility.

Some corridors which travel along industrial or manufacturing regions may have a greater percentage of trucks in their vehicle mix. The same is true around ports and intermodal facilities. In these instances, the truck drivers may not have an option for when they use the freeway system.